Stabilized hypobromous acid solutions

ABSTRACT

This invention relates to a method for preparing a stabilised stock hypobromous acid solution, and to a stabilised stock hypobromous acid solution. The stabilised stock solution has a hypobromous acid concentration of less than 30% (m/m) and contains an amount of cyanuric acid as a stabiliser not exceeding 1 ppm. The solution may be used for treating any type of water, but has particular application in treating drinking water and irrigation water.

BACKGROUND OF THE INVENTION

THIS invention relates to stabilised hypobromous acid solutions for thetreatment of water.

The use of hypobromous acid solutions in the treatment of industrialwaters and recreational waters is well known. Although hypobromous acidis a very effective general-purpose biocide, It is also very unstable.In some applications, hypobromous acid is made in situ by reacting abromide salt with a water-soluble hypochlorite such as sodiumhypochlorite. The solution so produced is added directly to water to betreated, before the hypobromous acid is no longer active.

U.S. Pat. No. 5,942,126, in the name of Nalco Chemical Company,discloses a sodium hypobromide solution that is stable under typicalstorage conditions. The hypobromite solution is stabilised with highconcentrations of a stabiliser which, inter alia, may be selected fromthe group consisting of urea, thiourea, creatinine, cyanuric acid, alkylhydantoins, mono or di ethanolamine, organic sulfonamides, biuret,sulfamic acid, organic sulfamates and melamine. This patent teaches theuse of this stabilised hypobromite solution in the treatment ofindustrial water systems.

It is an object of this invention to provide a stabilised hypobromousacid solution for the treatment/disinfection of water, especiallydrinking and irrigation water.

SUMMARY OF THE INVENTION

According to a first aspect of the invention there Is provided a methodfor preparing a stabilised stock hypobromous acid solution, for treatingwater, the method including the following steps:

-   -   1. preparing a hypochlorous acid solution with a pH of less than        7.5, preferably of 7.4;    -   2. preparing a bromide solution with a pH of less than 7.0,        preferably of 6.4;    -   3. mixing the hypochlorous acid solution with the bromide        solution to form a hypebromous acid solution; and    -   4. immediately adding a stabiliser to the solution to provide a        stabilised hypobromous acid solution with a pH of from 8 to 9,        preferably a pH of 8.8.

The hypochlorous acid solution of step 1 may be prepared by mixing ahypochlorite solution with a pH of about 14 with a hydrochloric acidsolution.

Typically, the stabiliser is cyanuric acid which, preferably, is addedin an amount not to exceed 1 ppm, advantageously not to exceed 0.5 ppm,in the hypobromous acid solution.

According to a second aspect of the invention there is provided astabilised stock hypobromous acid solution, for treating water, thesolution having a hypobromous acid concentration of less than 30% (m/m)typically less than 20% (m/m) and containing an amount of cyanuric acidas a stabiliser not exceeding 1 ppm.

Advantageously, the solution has a cyanuric acid concentration of lessthan 0.5 ppm.

Typically, the solution has a pH of 8 to 9, preferably a pH of 8.5 to8.9, most preferably a pH of 8.8.

Advantageously, stabilised solutions for drinking water have ahypobromous acid concentration of less than 10% (m/m).

Preferred stabilised solutions according to the invention for treatingdrinking water contain the following amounts of hypobromous acid andcyanuric acid:

-   -   1. A hypobromous acid concentration of 9% (m/m) and 0.2 ppm        cyanuric acid;    -   2. A hypobromous acid concentration of 6% (m/m) and 0.3 ppm        cyanuric acid; and    -   3. A hypobromous acid concentration of 3.5% (m/m) and 0.4 ppm        cyanuric acid.

A preferred stabilised solution for treating irrigation water will havea hypobromous acid concentration of from 10% to 20% (m/m).

The solution may be a sodium or potassium based hypobromous acidsolution, but for the purposes of drinking water and irrigation water,the solution is preferably a potassium based hypobromous acid solution.

Typically, a potassium based hypobromous acid solution has a potassiumconcentration of less than 20% (m/m), preferably less than 10% (m/m).

The preferred stabilised potassium based hypobromous solutions accordingto the invention for treating drinking water contain the followingamounts of hypobromous acid, potassium, and cyanuric acid:

-   -   1. a hypobromous acid concentration of 9% (m/m), a potassium        concentration of 3.7% (m/m) and 0.2 ppm cyanuric acid;    -   2. a hypobromous acid concentration of 6% (m/m), a potassium        concentration of 2.1% (m/m) and 0.3 ppm cyanuric acid; and    -   3. a hypobromous acid concentration of 3.5% (m/m), a potassium        concentration of 1.1% (m/m) and 0.4 ppm cyanuric acid.

The preferred stabilised potassium based hypobromous acid solutionsaccording to the invention for treating irrigation water contain thefollowing amounts of hypobromous acid, potassium, and cyanuric acid:

-   -   1. a hypobromous acid concentration of 13% (m/m), a potassium        concentration of 7% (m/m) and 0.4 ppm cyanuric acid;    -   2. a hypobromous acid concentration of 16% (m/m), a potassium        concentration of 8% (m/m) and 0.3 ppm cyanuric acid; and    -   3. a hypobromous add concentration of 18% (m/m), a potassium        concentration of 9% (m/m) and 0.2 ppm cyanuric acid.

According to a third aspect of the invention there is provided a methodof treating water, typically drinking or irrigation water, by adding astabilised solution of hypobromous acid as described above to the water.

Advantageously, sufficient of a solution of hypobromous add is added tothe water to provide a total bromine content of 0.5 to 0.001 mg/l in thewater.

Typically, the stabilised solution of hypobromous acid is added to thedrinking water in the filter trays of a water treatment plant.

Preferably, the free bromine residue of the treated water is monitoreddownstream of the water fixters of the water treatment plant and thestabilised solution of hypobromous acid is added at a dosage rate tomaintain a free bromine residue of 0.001 to 0.2 mg/l, preferably 0.025mg/l.

When used to treat irrigation water, the stabilised solution ofhypobromous acid may be added directly into irrigation lines, preferablyat the beginning of an irrigation water network.

Advantageously, the free bromine residue of the treated water in theirrigation network is monitored downstream in the network and thestabilised solution of hypobromous acid is added at a dosage rate tomaintain a free bromine residue of 0.001 to 0.2 mg/l, preferably 0.05mg/l.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the kill rate of a stabilised hypobromous acidsolution of the invention; and

FIG. 2 is a graph showing the decay rate of a 6% (m/m) hypobromous acidstabilised solution of the invention in comparison to a 15% (m/m)hypochlorous acid solution after the solution has been added to water inan amount of 6 mg/l.

DESCRIPTION OF EMBODIMENTS

This invention relates to a stabilised solution of hypobromous acid fortreating water, in particular for treating drinking water or irrigationwater. The stabilised solution is also suitable for treating other typesof water such as water in cooling towers and sewerage effluent.

As discussed in the background of the invention, although it is knownthat solutions of hypobromous acid are useful in destroyingmicro-organisms, hypobromous acid Is unstable and needs to bestabilised, in order to make it a commercially viable product.

The inventors have devised a new method for preparing a stockhypobromous acid solution and have, surprisingly, found out that thehypobromous acid solution so prepared may be stabilised with lowconcentrations, i.e. less than 1 ppm, preferably less than 0.5 ppm, ofcyanuric acid. Higher concentrations of cyanuric acid have been found toinhibit the action of bromine in destroying micro-organisms, and thushigher concentrations of cyanuric acid cause it to take longer for thehypobromous acid solution to be effective after application to water.

In accordance with the invention, the applicant has prepared stockhypobromous acid solutions containing 3.5%, 6% and 9% (m/m) hypobromousacid which are stabilised with cyanuric acid at a concentration of 0.4ppm, 0.3 ppm and 0.2 ppm, respectively. The concentration of hypobromousacid is determined by ion chromatography using a Dionex AD 14 ionexchange column, sodium carbonate—sodium bicarbonate as eluent andsuppressed conductivity detection. It has been found that thesesolutions are best. stabilised and most reactive at a pH of 8 to 9,preferably at a pH of 8.8. Solutions so stabilised have a shelf-life (ina sealed and light impervious container) of up to 6 months. It will alsobe seen that the inventors have, most surprisingly, found out that thereis an inverse relationship between the concentration of the cyanuricacid stabiliser and the concentration of the hypobromous acid solution.

In another aspect of the invention the inventors have found out thatstock solutions of stock hypobromous acid, particularly stabilisedsolutions as described above, may be used to treat drinking watereffectively. In accordance with the invention a stabilised stockhypobromous acid solution is added to drinking water to provide a totalbromine content of 0.5 to 0.001 mg/l in the water. The concentration ofhypobromous acid in the stock solution used is selected according to theorganic matter content in the drinking water. Water containing highamounts of organic matter will make use of a high concentration stock(i.e. the 9%) solution, and water containing low amounts of organicmaterial will make use of a low concentration stock (i.e. the 3.5%)solution.

In a normal drinking water treatment plant lime is added to the water toincrease pH from 7.9 to 8.4. A flocculant is then added to the water anda precipitate formed from the flocculant that settles out in clarifiers.A supernatant liquid (drinking water) from the clarifiers then flowsthrough a filtration system. The filtration system includes sandfilters. The drinking water from the clarifiers flows through thefilters under the force of gravity. Filtration speed is controlled bythe “head” on the filter and by way of outlet control valves. The flowrate of water through the filters depends on the filtration plantdesign, capacity and water pump. The water then flows from the filtersthrough filter trays. Usually, the water has a retention time in thefilter trays of 1 to 5 minutes. From the filter trays, water flows to asump, from where it is pumped to a reservoir. The drinking water thenflows, by way of booster pumps, from the reservoir to end users, forexample to homes. The homes can be from 3 km to 70 km (or further) awayfrom the water treatment plant.

In accordance with the invention, a stock solution of hypobromous acidis added to water in the water treatment plant, in the filter trays, byway of a venturi system or any other suitable system, for example dosingpumps. The stock solution is added from an inlet point below the watersurface, preferably at the bottom of the filter tray. Generally, asufficient amount of the stock solution is added to the water, toprovide a total bromine concentration of 0.001 to 0.5 mg/l in the water.In a preferred embodiment of the invention, a probe is placed in thewater line between the sump and the reservoir. The probe tests theoxidation reduction potential in the water, and the amount of stocksolution added is adjusted to provide a preferred concentration of thefree bromine of 0.025 to 0.1 mg/l. It is envisaged that the probe isconnected to a micro-processor which in turn is connected to a dosingsystem which manipulates the dosage of stock solution added to thewater, to provide a preferred concentration of free bromine. The probesends a reading back to a micro-processor. The probe activates orde-activates the dosing system based on the preset upper and lower setpoints in the event of equipment malfunction, an alarm is relayed to theoperators control room to prevent accidental dosing. The system can alsobe operated manually.

One of the major advantages of the stock solution according to theinvention is that it can be used to treat waters having a relativelywide pH range of 7.0 to 9. After the stock solution has been added towater which is to be treated, free bromine is formed (from thehypobromous acid) in the water. A formula for this reaction is providedbelow:OBr⁻+Br⁻+H₂O→Br₂+20H⁻.

It is the free bromine that destroys microorganisms in the water.Without wishing to be bound by theory, it is believed that the freebromine (formed from the hypobromous acid solution) damages thesemi-permeable cell membrane of the micro-organism, and either distortsthe structure of the cell membrane or breaks the cell membrane. When thestructure of the cell membrane is so distorted or broken, components ofthe organism within the cell leak out of the cell and the micro-organismdies. When drinking water is treated it is imperative that 99%, if notall of the micro-organisms are killed by the disinfectant that is addedto the water. If not, surviving microrganisms may, once the disinfectanthas lost its effect feed on the remains of micro-organisms that havebeen killed and proliferate in the “treated” water.

The free bromine is very effective in destroying micro-organisms. It hasbeen possible to obtain a 99% kill rate of micro-organisms in watertreated with solutions according to the invention within 60 seconds ofadding the stabilised solution to the water. FIG. 1 shows the results ofa test conducted to show the effectiveness of a 6% (m/m) hypobromousacid of the solution on bacteria in a water sample. 0.07 mg/l of the 6%(m/m) hypobromous acid solution was added to a water sample containing20 000 total bacteria species per ml. The total bacteria count was madeat time intervals after adding the hypobromous acid solution. The graphclearly shows that 99% of the bacteria are killed within 50 seconds ofadding the solution to the water.

Another advantage of the stabilised solution according to the inventionis that the activity of the bromine, after being added to the drinkingwater, is short lived in fact, there is less than 0.2 mg/l, typicallyabout 0.025 mg/l of free bromine residue in the water with 45 minutes ofbeing added to the water, as can be seen in FIG. 2. This means thatwater treated in accordance with the stabilised solution of theinvention can almost immediately (after 45 minutes) be used by consumersas there is a very low bromine concentration (typically about 0.025 mg/lfree bromine residue). The free chlorine, on the other hand, remains inthe water for a long period of time.

A further advantage of the stabilised stock solution of the invention isthat the low free bromine residue of from 0.001 mg/l to 0.2 mg/l,typically about 0.025 mg/l, is maintained in the water for long periodsof time, and until the treated water reaches an end user. For examplethese residues can remain for the time that the treated water is storedin a reservoir, and subsequently pumped 70 km in a water distributionnetwork to the end user. Albeit low, this free bromine residue issufficient to prevent the proliference of microorganisms in the waterfrom the time that it is treated until it reaches the end user.

Thus, because of the fast kill rate, when a hypobromous acid solutionaccording to the invention is added in the filter trays of a watertreatment plant, 99% of micro-organisms in the water are killed beforethe water leaves the filter tray. And, once the drinking water reachesthe end user, there are low concentrations of free bromine in the water.Furthermore, the low concentrations of free bromine in the water ensurethat there is no growth of micro-organisms before the drinking waterreaches the end user. This is very different to the treatment of waterwith chlorine, where high amounts of available chlorine are stillpresent in the drinking water, once it has reached the end user. Itshould also be mentioned that because such low concentrations ofcyanuric acid are used in the stock solutions, once added to thedrinking water, the concentration of cyanuric acid in the treateddrinking water is negligible, and thus will have no effect on end users.

The hypobromous acid stock solutions according to the invention areproduced by combining a solution containing bromide ions with a solutioncontaining hypochlorous acid.

The solution containing bromide ions may be formed by dissolving abromide source in water. The bromide source may be selected from sodiumbromide, potassium bromide or lithium bromide. According to a firstembodiment of the invention the bromide ion solution is formed bydissolving sodium bromide in water, to provide a 37% solution.

The hypochlorous acid solution may be prepared from an alkali oralkaline earth metal hypochlorite selected from sodium hypochlorite,potassium hypochlorite, magnesium hypochlorite, lithium hypochlorite andcalcium hypochlorite. Usually, such a solution has a pH of about 14. ThepH of the solution is lowered to below 7.5, by adding hydrochloric acid,to provide a hypochlorous acid solution preferably with a pH of 7.41,and 3.5%, by weight, available chlorine.

The hypochlorous acid and bromide solutions are then combined inquantities to provide the required concentration of hypobromous acid. Aformula for this reaction is set out below:HOCl+NaBr→HOBr+NaCl

For example, the preferred bromide solution described above is combinedwith the preferred hypochlorous acid solution described above at a ratioof 1:7.4 of bromide solution to hypochlorous acid solution to provide a3.5% (m/m) hypobromous acid solution, or combined at a ratio of 1:3.7 ofbromide solution to hypochlorous acid solution to provide a 6% (m/m)hypobromous acid solution, or combined at a ratio of 1:1.89 of bromidesolution to hypochlorous acid solution to provide a 9% (m/m) hypobromousacid solution.

A stabiliser, in the form of cyanuric acid (dissolved in water which hasbeen heated to 40° C.) is then added immediately to the hypobromous acidsolution so formed. A small amount of the cyanuric acid, i.e. less than1 ppm, preferably less than 0.5 ppm is added. In a preferred embodimentof the invention 0.4 ppm cyanuric acid is added to a 3.5% hypobromousacid solution, 0.3 ppm to a 6% hypobromous acid solution and 0.2 ppm toa 9% hypobromous acid solution.

The stock solutions so produced have been found to have a shelf life ofup to six months when stored in a sealed container that is impervious tolight.

The hypobromous acid solutions as described above are sodium basedsolutions, i.e. they are formed by the reaction of hypochlorous acidwith a solution of sodium bromide. A problem with the use of a sodiumbased hypobromous acid solution for treating irrigation water is thatthe sodium can lead to soil salination. Indeed, the sodium adsorptionratio (SAR) is an index of the potential of a given irrigation water toinduce sodic soil conditions (soil sodicity is usually measured by thepercentage of a soil's caflon exchange capacity that is occupied bysodium ions). It is calculated from the concentrations of sodium,calcium and magnesium in water and gives an indication of the level atwhich the exchangeable sodium percentage (ESP) of the soil willstabilise after prolonged irrigation. If the SAR of water is too high,this will cause salinity of the soil. The salinity has a negative effecton roots of plants in the soil and negatively effects the uptake ofimportant micro-elements such as calcium and magnesium.

It has been found that, although more expensive to produce, it isbeneficial to use potassium based hypobromous acid stabilised solutions,i.e. solutions formed by the reaction of hypochlorous acid with asolution of potassium bromide. These solutions are also stabilised atlow concentrations, i.e. less than 1 ppm, preferably less than 0.5 ppmcyanuric acid. Again, higher concentrations of cyanuric acid have beenfound to inhibit the action of the bromine in destroyingmicro-organisms, and thus higher concentrations of cyanuric acid causeit to take longer for the hypobromous acid solution to be effective.

In accordance with the invention, the applicant has preparedpotassium-based stabilized hypobromous acid concentrate solutions fortreating irrigation water containing 13% (m/m) hypobromous acid and 7%(m/m) potassium, 1.6% (m/m) hypobromous acid and 8% (m/m) potassium, and18% (m/m) hypobromous acid and 9% (m/m) potassium, which are stabilisedwith cyanuric acid at a concentration of 0.4 ppm, 0.3 ppm and 0.2 ppm,respectively. The potassium concentration is determined usinginductively coupled plasma spectrometry (ICP). Solutions so stabilisedhave a shelf-life tin a sealed and light impervious container) of up tosix months. It has also been found that the solutions are beststabilised and most reactive at a pH of 8 to 9, preferably at a pH of8.8.

The abovementioned potassium based stabilised stock hypobromous acidsolutions are added directly into the irrigation lines to provide atotal bromine content in the irrigation water to 0.001 mg/l to 0.5 mg/lin the water. Again, the concentration of the hypobromous acid in thestock solution used is selected according to the organic matter presentin the irrigation water irrigation water containing high amounts oforganic matter will make use of a high concentration stock (i.e. the18%) solution, and irrigation water containing lower amounts of organicmaterial will make use of a low concentration stock (i.e. the 13%)solution.

Usually, the solution of potassium based stabilised hypobromous acidconcentrate is added to the irrigation water at the beginning ofdistribution into an irrigation water network. The free bromineconcentration of the treated water is monitored downstream and thesolution of potassium based stabilised hypobromous acid is added at adosage rate to maintain a free bromine residue of 0.001 to 0.2 mg/l,typically 0.02 to 0.1 mg/l, preferably 0.05 mg/l.

As with the sodium based hypobromous acid solution, the potassium basedhypobromous acid solution is active over a wide pH range of 7 to 9, andit obtains a 99% kill rate of micro-organisms in the irrigation waterwithin 60 seconds of being added to the water. The activity of thebromine is also relatively short lived and there is less than 0.02 mg/lof active bromine in the irrigation water within 45 minutes of beingadded to the water. Thus, because of the fast kill rate, when apotassium based hypobromous acid solution according to the invention isadded directly to the waterlines of an irrigation network, 99% ofmicro-organisms are killed while the water is being distributed to thenetwork and, by the time the irrigation water reaches the end of thenetwork (and thus the soil), there are low concentrations (less than 0.2mg/l, typically about 0.01 to 0.1 mg/l) of free bromine (residue) in thewater.

A further advantage of potassium based hypobromous acid solutions isthat potassium is an important micro-element for plant growth and thishas the effect of fertilising the soil to which it is applied.

As mentioned above, although the potassium based hypobromous acidsolutions according to the invention find particular use in irrigationwater, they may also be used for treating drinking water, and othertypes of water. It is also believed that the potassium added to drinkingwater is beneficial for human consumers. Such potassium basedhypobromous solutions may also be preferred for treating sewerage wateras this solution has a better affect on the environment than a sodiumbased hypobromous acid solution.

Typical potassium based stabilised stock hypobromous acid solutions fortreating drinking water contain 3.5% (m/m) hypobromous acid and 1.1%(m/m) potassium, 6% (m/m) hypobromous acid and 2.1% (m/m) potassium, and9% (m/m) hypobromous acid and 3.7% (m/m) potassium, which are stabilisedwith cyanuric acid at a concentration of 0.4 ppm, 0.3 ppm and 0.2 ppm,respectively. Solutions so stabilised have a shelf-life (in a sealed andlight impervious container) of up to six months. Again, the surprisinginverse relationship between the concentration of cyanuric acidstabiliser and the concentration of the hypobromous acid solution isevident. It has also been found that the solutions are best stabilisedand most reactive at a ph of 8 to 9, preferably at a pH of 8.8.

The above-mentioned potassium based stabilised hypobromous acidsolutions may be added to drinking water (in the manner described above)to provide a total bromine content of 0.001 mg/l to 0.5 mg/l in thewater.

The potassium based hypobromous acid solutions according to theinvention are produced by combining a solution containing potassiumbromide ions with a solution containing hypochlorous acid.

The solution containing bromide ions may be formed by dissolvingpotassium bromide in water. According to a preferred embodiment of theinvention the bromide ion solution is formed by dissolving potassiumbromide in water, to provide a 30% (m/m) solution.

The hypochlorous acid solution may be prepared from an alkali oralkaline earth metal hypochlorite solution selected from sodiumhypochlorite, potassium hypochlorite, magnesium hypochlorite, lithiumhypochlorite and calcium hypochlorite. Usually, such a solution has a pHof about 14.5. The pH of the solution is lowered to about 7.5 by addinghydrochloric acid, to provide a hypochlorous acid solution preferablywith a pH of 7.41 and 3.5%, by weight, available chlorine.

The hypochlorous acid and potassium bromide solutions are then combinedin quantities to provide the required concentration of a potassium basedhypobromous acid. A formula for this reaction is set out below:HOCl+KBr→HOBr+KCl

For example, in order to prepare stabilised solutions for treatingirrigation water, the preferred potassium based bromide solutiondescribed above is combined with the preferred hypochlorous acidsolution described above at a ratio of 7.4:1 to provide a 18% (m/m)potassium based hypobromous acid solution with 9% (m/m) potassium, orcombined at a ratio of 3.7:1 to provide a 16% (m/m) potassium basedhypobromous acid solution with 8% (m/m) potassium, or combined at aratio of 1.89:1 to provide a 13% (m/m) potassium based hypobromous acidsolution with 7% (m/m) potassium.

A stabiliser, in the form of cyanuric acid (dissolved in water which hasbeen heated to 40° C.) is then added immediately to the potassium basedhypobromous acid solution so formed. A small amount of the cyanuricacid, i.e. less than 1 ppm, preferably less than 0.5 ppm is added.Sufficient cyanuric acid is added to provide 0.2 ppm cyanuric acid inthe 18% (m/m) potassium based hypobromous acid solution, 0.3 ppm in the16% (m/m) hypobromous acid solution and 0.4 ppm cyanuric acid in the 13%(m/m) potassium based hypobromous acid solution.

The stock solutions so produced have been found to have a shelf life ofup to six months when stored in a sealed container that is impervious tolight.

EXAMPLE 1 Preparation of Stabilised Stock Sodium Based Hypobromous AcidSolutions for Use in Treating Drinking Water

132.5 l of a sodium hypochlorite solution having 15% availablehypochlorite, at a pH of 12.7, was mixed with 365.5 l of water and thepH of this solution is lowered to 7.41 by adding 14.6 g/l ofhydrochloric acid (10%), to provide a hypochlorous acid stock solutionhaving a free chlorine content of 3.5%, by weight.

185 kg of sodium bromide was dissolved in 315 l of water to provide a37%, by weight, sodium bromide stock solution having a pH of 6.4.

EXAMPLE 1A 3.5% Stabilised Sodium Based Hypobromous Acid Solution

A 3.5% (m/m) hypobromous acid solution according to the invention wasprepared by mixing 500.24 l of the hypochlorous acid stock solutiondescribed above with 67.6 l of the sodium bromide stock solutiondescribed above (i.e. the sodium bromide and hypochlorous acid solutionsmentioned above are mixed at a ratio of 1:7.4), to form a solutioncontaining 3.5% (m/m) hypobromous acid at a pH of 8.8. 227.14 mg ofcyanuric acid (dissolved in water heated to 40° C.) is then addedimmediately to the solution to provide a concentration of cyanuric acidof 0.4 ppm.

EXAMPLE 1B 6% Stabilised Sodium Based Hypobromous Acid Solution

A 6% (m/m) hypobromous acid solution according to the invention wasprepared by mixing 500.02 l of the hypochlorous acid stock solutiondescribed above with 135.14 l of the sodium bromide stock solutiondescribed above (i.e. the sodium bromide and hypochlorous add solutionsmentioned above are mixed at a ratio of 1:3.7), to form a solutioncontaining 6% (m/m) hypobromous acid at a pH of 8.8. 190.55 mg ofcyanuric acid (dissolved in water heated to 40° C.) is then addedimmediately to the solution to provide a concentration of cyanuric acidof 0.3 ppm.

EXAMPLE 1C 9% Stabilised Sodium Based Hypobromous Acid Solution

A 9% (m/m) hypobromous acid solution according to the invention wasprepared by mixing 500 l of the hypochlorous acid stock solutiondescribed above with 264.55 l of the sodium bromide stock solutiondescribed above (i.e. the sodium bromide and hypochlorous acid solutionsmentioned above are mixed at a ratio of 1:1.89), to form a solutioncontaining 9% (m/m) hypobromous acid at a pH of 8.8. 152.91 mg ofcyanuric acid (dissolved in water heated to 40° C.) is then addedimmediately to the solution to provide a concentration of cyanuric acidof 0.2 ppm.

EXAMPLE 2 Preparation of Stabilised Stock Potassium Based HypobromousAcid Solutions

132.5 l of a sodium hypochlorite solution having 15% available aschlorine, at a pH of 14.5 was mixed with 365.5 l of water to provide ahypochlorous acid, solution with a pH of 14.2. The pH of this solutionis lowered to 7.5 by adding 14.6 g/l of hydrochloric acid (10%), toprovide a hypochlorous acid stock solution having a free chlorinecontent of 3.5% by weight.

150 kg of potassium bromide was dissolved in 350 l of water to provide a30%, by weight, potassium bromide stock solution having a pH of 6.9.

EXAMPLE 2A 18% Stabilised Potassium Based Hypobromous Acid Solution

A 18% (m/m) potassium based hypobromous acid solution according to theinvention was prepared by mixing 15.91 l of the hypochlorous acid stocksolution described above with 117.74 l of the potassium bromide stocksolution described above. i.e., the potassium bromide and hypochlorousacid solution mentioned above are mixed at a ratio of 7.4:1 to form asolution containing 18% (m/m) hypobromous acid at a pH of 8.8 and 9%(m/m) potassium. 26.73 mg of cyanuric acid (dissolved in water heated to40° C.) is then added immediately to the solution to provide aconcentration of cyanuric acid at 0.2 ppm.

EXAMPLE 2B 16% Stabilised Potassium Based Hypobromous Acid Solution

A 16% (m/m) potassium based hypobromous acid solution according to theinvention was prepared by mixing 57.84 l of the hypochlorous acid stocksolution described above with 214.01 l of the potassium bromide stocksolution as described above. (i.e. the potassium bromide andhypochlorous acid solution mentioned above are mixed at a ratio of3.7:1). To form a solution containing 16% (m/m) hypobromous acid at a pHof 8.8 and 8% (m/m) potassium. 81.56 mg of cyanuric acid (dissolved inwater heated to 40° C.) is then added immediately to the solution toprovide a concentration of cyanuric acid of 0.3 ppm.

EXAMPLE 2C 13% Stabilised Potassium Based Hypobromous Acid Solution

A 13% (m/m) potassium based hypobromous acid solution according to theinvention was prepared by mixing 113.23 l of the hypochlorous acid stocksolution described above with 214.01 l of the sodium bromide stocksolution, described above. (i.e. the potassium bromide and hypochlorousacid stock solution mentioned above are mixed at a ratio of 1.89:1 toform a solution containing 13% (m/m) hypobromous acid at a pH of 8.8 and7% (m/m) potassium. 130.9 mg of cyanuric acid (dissolved in water heatedto 40° C.) is then added immediately to the solution to provide aconcentration of cyanuric acid of 0.4 ppm.

EXAMPLE 3 Preparation of Stabilised Stock Potassium Based HypobromousAcid Solutions for Use in Treating Drinking Water

132.51 of a sodium hypochlorite solution having 15% availablehypochlorite, at a pH of 12.7, was mixed with 365.5 l of water and thepH of this solution is lowered to 7.41 by adding 14.6 g/l ofhydrochloric acid (10%), to provide a hypochlorous acid stock solutionhaving a free chlorine content of 3.5%, by weight.

150 kg of potassium bromide was dissolved in 350 l of water to provide a30%, by weight, potassium bromide stock solution having a pH of 6.4.

EXAMPLE 3A 3.5% Stabilised Potassium Based Hypobromous Acid Solution

A 3.5% (m/m) potassium based hypobromous acid solution according to theinvention was prepared by mixing 117.74 l of the hypochlorous acid stocksolution described above with 15.91 l of the potassium bromide stocksolution described above (i.e. the potassium bromide and hypochlorousacid solutions mentioned above are mixed at a ratio of 1:7.4), to form asolution containing 3.5% (m/m) hypobromous acid and 1.1% (m/m) potassiumat a pH of 8.8. 53.46 mg of cyanuric acid (dissolved in water heated to40° C.) is then added immediately to the solution to provide aconcentration of cyanuric acid of 0.4 ppm.

EXAMPLE 3B 6% Stabilised Potassium Based Hypobromous Acid Solution

A 6% (m/m) stabilised potassium based hypobromous acid solutionaccording to the invention was prepared by mixing 214.01 l of thehypochlorous acid stock solution described above with 57.84 l of thepotassium bromide stock solution described above (i.e. the potassiumbromide and hypochlorous acid solutions mentioned above are mixed at aratio of 1:3.7), to form a solution containing 6% (m/m) hypobromous acidand 2.1% (m/m) potassium at a pH of 8.8. 81.56 mg of cyanuric acid(dissolved in water heated to 40° C.) is then added immediately to thesolution to provide a concentration of cyanuric acid of 0.3 ppm.

EXAMPLE 3C 9% Stabilised Potassium Based Hypobromous Acid Solution

A 9% (m/m) potassium based stabillsed hypobromous acid solutionaccording to the invention was prepared by mixing 214.01 l of thehypochlorous acid stock solution described above with 113.23 l of thepotassium bromide stock solution described above (i.e. the potassiumbromide and hypochlorous acid solutions mentioned above are mixed at aratio of 1:1.89), to form a solution containing 9% (m/m) hypobromousacid and 3.7% (m/m) potassium at a pH of 8.8. 65.45 mg of cyanuric acid(dissolved in water heated to 40° C.) is then added immediately to thesolution to provide a concentration of cyanuric acid of 0.2 ppm

1. A method for preparing a stabilized stock hypobromous acid solutionincluding the following steps: preparing a hypochlorous acid solutionwith a pH of less than 7.5; preparing a bromide solution with a pH ofless than 7.0; mixing the hypochlorous acid solution with the bromidesolution to form a hypobromous acid solution; and immediately adding astabilizer to the solution to provide a stabilized stock hypobromousacid solution with a pH of from 8 to
 9. 2. A method according to claim 1wherein the hypochlorous acid solution has a pH 7.4.
 3. A methodaccording to claim 2 wherein the bromide solution has a pH of 6.4.
 4. Amethod according to claim 3 wherein the stabilized hypobromous acidsolution has a pH of 8.8.
 5. A method according to any one of claims 1to 4 wherein the stabilizer is cyanuric acid.
 6. A method according toclaim 5 wherein the cyanuric acid is added in an amount not to exceed 1ppm in the hypobromous acid solution.
 7. A stabilized stock hypobromousacid solution having a hypobromous acid concentration of less that 30%(m/m) and containing an amount of cyanuric acid as a stabilizer notexceeding 1 ppm.
 8. A stabilized stock hypobromous acid solutionaccording to claim 7 having a hypobromous acid concentration of lessthan 20% (m/m).
 9. A stabilized stock hypobromous acid solutionaccording to claim 7 or 8 containing less than 0.5 ppm cyanuric acid.10. A stabilized stock hypobromous acid solution according to claim 7 or8 having a pH of 8 to
 9. 11. A stabilized stock hypobromous acidsolution according to claim 10 having a pH of 8.5 to 8.9.
 12. Astabilized stock hypobromous acid solution according to claim 11 havinga pH of 8.8.
 13. A stabilized stock hypobromous acid solution accordingto claim 7 or 8 for drinking water, having a hypobromous acidconcentration of less than 10% (m/m).
 14. A stabilized stock hypobromousacid solution according to claim 13 having a hypobromous acidconcentration of 9% of (m/m) and containing 0.2 ppm cyanuric acid.
 15. Astabilized stock hypobromous acid solution according to claim 13 havinga hypobromous acid concentration of 6% (m/m) and containing 0.3 ppmcyanuric acid.
 16. A stabilized stock hypobromous acid solutionaccording to claim 13 having a hypobromous acid concentration of 3.5%(m/m) and containing 0.4 ppm cyanuric acid.
 17. A stabilized stockhypobromous acid solution according to claim 7 or 8 for treatingirrigation water having a hypobromous acid concentration of from 10% to20% (m/m).
 18. A stabilized stock hypobromous acid solution according toclaim 7 or 8 which is potassium based having a potassium concentrationof less than 20% (m/m).
 19. A stabilized potassium based stockhypobromous acid solution according to claim 18, for treating drinkingwater, having potassium concentration of less than 10% (m/m).
 20. Astabilized potassium based stock hypobromous solution according to claim19 having a hypobromous acid concentration of 9% (m/m), a potassiumconcentration of 3.7% (m/m) and containing.
 21. A stabilized potassiumbased stock hypobromous solution according to claim 19 having ahypobromous acid concentration of 6% (m/m), a potassium concentration of2.1% (m/m) and containing 0.3 ppm cyanuric acid.
 22. A stabilizedpotassium based stock hypobromous solution according to claim 19 havinghypobromous acid concentration of 3.5% (m/m), a potassium concentrationof 1.1% (m/m) and containing 0.4 ppm cyanuric acid.
 23. A stabilizedpotassium based stock hypobromous acid solution according to claim 18,for treating irrigation water, having a hypobromous acid concentrationof 13% (m/m), a potassium concentration of 7% (m/m) and containing 0.4ppm cyanuric acid.
 24. A stabilized potassium based stock hypobromousacid solution according to claim 18, for treating irrigation water,having a hypobromous acid concentration of 16% (m/m), a potassiumconcentration of 8% (m/m) and containing 0.3 ppm cyanuric acid.
 25. Astabilized potassium based stock hypobromous acid solution according toclaim 18, for treating irrigation water; having a hypobromous acidconcentration of 18% (m/m), a potassium concentration of 9% (m/m) andcontaining 0.2 ppm cyanuric acid.
 26. A method of treating water byadding a stabilized stock hypobromous acid solution as defined in claim7 or 8 to the water.
 27. A method of treating drinking water by adding astabilized stock hypobromous acid solution as defined in claim or 8 tothe water.
 28. A method of treating irrigation water by adding astabilized stock solution of hypobromous acid as defined in claim 7 or 8to the water.
 29. A method according to, any one of claims 26 to 28wherein sufficient of the stabilized stock solution of hypobromous acidis added to the water to provide a total bromine content to 0.5 to 0.001mg/l in the water.
 30. A method according to claim 27 wherein thestabilized stock solution of hypobromous acid is added to the drinkingwater in the filter trays of a water treatment plant.
 31. A methodaccording to claim 30 wherein the free bromine residue of the treatmentwater is monitored downstream of the water filters of the watertreatment plant and the stabilized stock of hypobromous acid is added ata dosage rate to maintain a free bromine residue of 0.001 to 0.2 mg/l.32. A method according to claim 31 wherein the free bromine residue ofthe treatment water is monitored downstream of the water of watertreatment plant and the stabilized stock solution of hypobromous acid isadded at a dosage rate to maintain a free bromine residue of 0.025 mg/l.33. A method according to claim 28 wherein stock solution of hypobromousacid is added directly into irrigation lines of an irrigation waternetwork.
 34. A method according to claim 33 wherein the stabilized stocksolution of hypobromous acid is added at the beginning of the irrigationwater network.
 35. A method according to claim 34 wherein the freebromine residue of the treated water in the irrigation network ismonitored downstream in the network and the stabilized solution ofhypobromous acid is added at a dosage rate to maintain a free bromineresidue of 0.001 to 0.2 mg/l.
 36. A method according to claim 35 whereinthe free bromine residue of the treated water in the irrigation networkis monitored downstream in the network and the stabilized solution ofhypobromous acid is added at a dosage rate to maintained a free bromineresidue of 0.05 mg/l.